KAYSTROS (or Cayster) was a River-God of Lydia in Anatolia (modern Turkey).

The River Kaystros had its headwaters on Mount Tmolos and flowed into the Aegean Sea near Ephesos on the Lydian border with Karia. The next major rivers were the Meles and Hermos in the north, and Maiandros to the south.

Strabo, Geography 14. 1. 44 (trans. Jones) (Greek geographer C1st B.C. to C1st A.D.) :
"And not far from Leimon [in Karia] is . . . a hero-temple of Kaystros and a certain Asios, and the Kaystros River that streams forth near by."

Pausanias, Description of Greece 7. 2. 8 (trans. Jones) (Greek travelogue C2nd A.D.) :
"Ephesos, who is thought to have been a son of the river Kaystros, and from Ephesos the city received its name."

Philostratus the Elder, Imagines 1. 11 (trans. Fairbanks) (Greek rhetorician C3rd A.D.) :
"Swans scattered about, breathing sweet notes, will hymn the youth [i.e. Phaethon, who was cast from the chariot of the sun]; and flocks of swans rising aloft will sing the story to [the Rivers] Kaÿstros and Istros; nor will any place fail to hear the strange story."